1. Field of the Invention
The invention relates generally to a method and apparatus for removing fine particulate pollutants from stack gases and more particularly to an improved method and apparatus for draining the centrifugal fans used in such a system.
2. Description of the Prior Art
In the manufacture of glass fiber wool-type products, molten glass from a melting and refining tank flows into and through a centrifuge forming step downwardly onto a foraminous belt. As the glass fibers are falling onto the belt, a cureable organic resin binder is sprayed into the stream of fibers. The foraminous belt conveys the fiber and resin mixture through a curing oven, wherein the resin is cured to bind the fibers into a wool-like product.
Located beneath the foraminous conveyor is an air intake chute into which air is induced from the falling fiber-resin mixture, along with induced factory air. This air flow is induced by one or more fans (three in the present embodiment) pulling air through the chute and discharging air into an upper fallout chamber, de-entrainent device or "penthouse" from which the air passes into a vertical exhaust stack for flow into the ambient atmosphere.
The fans may be quite large and huge volumes of air, normally on the order of 50,000 to 150,000 standard cubic feet per minute (scfm) for the total fan system flow through the chute directly beneath the glass fiber supporting belt. This induced air contains miscellaneous solid and liquid particles, e.g. glass fibers, unreacted phenol and aldehyde components of the resin, uncured liquid resin, factory air-entrained solids and calcium sulfate, phosphate, or carbonate which are precipitates of the catalysts normally used for the phenol formaldehyde resin. The particulate content of this air is on the order of 0.400 grains per scf. According to typical Environmental Protection Agency (EPA) standards, the maximum allowable effluent from the stack is 0.020 grains per scf, so removal of 95% of the particulate content is necessary to comply with the EPA regulations. While the EPA standards may vary from state to state, the above example is an actual typical EPA value.
Prior to the present invention, the particulates in the forming air exhaust have been treated primarily by the use of "dropout boxes". Such boxes are interposed between the forming conveyor and the fan, and water is sprayed into the box. Due to the large cross-sectional area of the box and the water sprayed thereinto, large particulate particles drop out in the box through a combination of water impingement with the particulate and settling of the particulate from the slowly moving air traveling through the box. As a result, the total particulate count at the outlet of the box drops to about 0.040 grains per scf. Although 90% of the particulates have been removed by use of the dropout box, the remaining particulates are all fine particles, in the nature of aerosols, which follow the air flow through the forming fan and the final settling penthouse. It is these particles in the stack gas which make up the chemical plume or haze issuing from the stacks in typical glass wool forming operations.
It is necessary that these fine particulates be removed from the forming air in order to conform with the various federal, state and local EPA regulations. The present invention utilizes massive quantities of water sprayed into the forming fan to insure compliance with these regulations. The aerosol particles are agglomerated and coalesced by impact between the particles and the water droplets as well as between the particles and the wet surfaces of the fan and the fan shroud or hood. The fan may be provided with a venturi throat inlet and massive amounts of water, on the order of 1.5 gallons per minute (gpm) per thousand standard cubic feet per minute (scfm) of forming air per fan, are introduced into the fan inlet as a spray of water droplets.
The massive quantities of water which are injected into the forming fan are essential for the efficient operation of the present system in order to achieve optimum particulate removal. The centrifugal fans of the prior art were unable to continually handle these massive quantities of water due to the tendency of the fan housing to partially fill with water.
The present invention provides a method and apparatus for removing fine particulate pollutants from stack gases by injecting massive volumes of wash water into a centrifugal fan receiving the stack gases and positively stripping these massive volumes of wash water from the fan housing to achieve maximum particulate removal.